Technical Abstract:
Bacterial wilt, caused by Ralsotnia Solanacearum, is an extremely damaging disease of flue cured tobacco. Bacterial wilt losses in South Carolina, expressed as the percentage of total crop production, have increased from 0.2% in 1981 to 7.2% in 1998. Consolidation of tobacco allotments from 24,000 in 1981 to <5000 in 1998 has resulted in larger farming units, and increased mechanization in flower (topping) and leaf removal (mechanical leaf harvesters).
The role of mechanical topping in the spread of bacterial wilt was evaluated in randomized complete block factorial experiments on the Pee Dee Research and Education Center and in large-scale on-farm trials. Main blocks were method of flower removal (hand vs. mechanical) and subplots were tobacco cultivar (K 326 vs. K 346). Mechanical flower removal spreads R. solanacearum from infected plant tissue to healthy plants (P<0.001) increasing the incidence and severity of disease. Host resistance (K346) did not reduce the spread of the disease through mechanical topping when compared to the susceptible control (K 326, P=0.05).
The role of mechanical leaf harvesting on the spread of bacterial wilt was evaluated in randomized complete block factorial experiments. Main blocks were method of leaf removal (hand removal, simulated mechanical removal and simulated mechanical removal with slight injury to the stalk) and subplots were tobacco cultivar (K326 vs. K346). The bacterium was mechanically transmitted to the healthy plants during hand and mechanical harvesting (P<0.001). Transmission of the bacterium and infection of tobacco plants following leaf removal with R. solanacearum-contaminated hands or harvesting aids ranged from 47% to 83% of the planting (hand vs. mechanical harvesting, respectively). Infected plants remained symptomless for 3-6 weeks following infection and then wilted rapidly. Host resistance (K346) did not reduce the spread of the disease through stalk injury (leaf removal) when compared to a susceptible control (K 326, P=0.05). The presence of R. solanacearum was confirmed with a tetrazolium based selective medium, FAME analysis, and a tomato bioassay.